1. Field of the Invention
The present invention relates to a bidirectional optical recirculation loop transmitting device, and more particularly, to a bidirectional optical recirculation loop transmitting device capable of long-distance transmission using a small number of test samples in a wavelength division multiple optical transmission system.
2. Description of the Related Art
FIG. 1 shows a configuration of a conventional unidirectional optical recirculation loop transmitting device. Referring to FIG. 1, the conventional unidirectional optical recirculation loop transmitting device includes a transmitting unit 100 for generating and transmitting an N-number of optical signals, a wavelength division multiplexer (WDM) 110 for multiplexing the N-number of optical signals, a first optical amplifier 121 for amplifying multiplexed optical signals, a second optical switch 132 for selectively passing an optical signal output from the first optical amplifier under the control of a controller 190 that is described later, an optical combiner 140 for separating an optical signal output from the second optical switch 132 into two optical signals and outputting the separated optical signals, an optical link 150 that is a path along which one of the separated optical signals is transmitted, a first optical switch 131 for selectively passing the optical signal transmitted along the optical link 150 under the control of the controller 190, a second optical amplifier 122 for amplifying the other one of the two optical signal separated from the optical combiner 140, a wavelength division demultiplexer 160 for separating an optical signal output from the second optical amplifier 122 according to the wavelength of each channel, a receiving unit 170 for receiving an optical signal output from the wavelength division demultiplexer 160, a measuring instrument 180 connected to the receiving unit 170 and monitoring performance of an optical signal per channel of the unidirectional optical recirculation loop transmitting device, and the controller 190 for controlling the first optical switch 131, the second optical switch 132, and the measuring instrument 180.
The transmitting unit 100 consists of an N-number of transmitters from a first transmitter 1001 to an Nth transmitter 100N. The receiving unit 170 consists of an N-number of receivers from a first receiver 1701 to an Nth receiver 170N. The optical link 150, which is a transmission path of an optical signal, consists of a plurality of nodes and optical fibers.
FIG. 2 is a time diagram with respect to the first optical switch signal, the second optical switch signal, an optical recirculation loop output signal, and a gate trigger signal of FIG. 1. Referring to FIG. 2, the first optical switch 131 and the second optical switch 132 operate in the opposite states. That is, from a time point at which t=0 to a time point at which t=1T, the second optical switch 132 is in an “ON” state while the first optical switch 131 is in an “OFF” state. Also, from a time point at which t=1 T to a time point at which t=nT, the second optical switch 132 is in the “OFF” state while the first optical switch 131 is in the “ON” state. It can be seen that a period when the second optical switch 132 remains in the “ON” state is equivalent to a time T needed for the optical signal to proceed in the optical link 150 in the optical recirculation loop.
When the second optical switch 132 is in the “ON” state from the time point at which t=0 to the time point at which t=1T, the optical signal output from the transmitting unit 100 is input to a first port of the optical combiner 140 having four ports: half of the input optical signal is transmitted to the receiving unit 170 via a third port and the other half is transmitted to the optical link 150 via a fourth port. An optical signal equivalent to the time T output from the transmitting unit 100 from the time point at which t=0 to the time point at which t=1 T is transmitted to the receiving unit 170 and the optical signal equivalent to the time T remains in the optical link 150.
When the second optical switch 132 is in the “OFF” state and the first optical switch 131 is in the “ON” state at the time point at which t=1T, the optical signal passing through the optical link 150 is input to a second port of the optical combiner 140 having four ports: half of the input optical signal is transmitted to the receiving unit 170 via the third port and the other half is transmitted to the optical link 150 again via the fourth port.
According to the same operation principle, as time passes, optical signals having traveled a longer distance are sequentially received by the receiving unit 170. As shown in FIG. 2, the receiving unit 170 can receive an optical signal transmitted after rotating an n−1 turn.
When the length of the optical link 150 is M km, the receiving unit 170 can sequentially receive an optical signal transmitted 0 km to (n−1)M km. Since the optical signals sequentially arrive at the receiving unit 170, in FIG. 2, the optical intensity at the receiving unit 170 represented by an optical recirculation loop output is detected to indicate that an optical signal is always present.
When performance of an optical signal corresponding to a desired transmission distance is to be measured by the measuring instrument 180 connected to the receiving unit 170, only a range of the optical signals corresponding to the number of rotations needs to be detected from the sequential signals. To do so, only a portion corresponding to a particular rotation number k is gated like the gate trigger in FIG. 2 to use only a value from a time point at which t=kT to a time point at which t=(k+1)T as a measurement material while the other portion is excluded from the measurement material. Since there may be a contaminated optical signal at around a boundary region of the rotation number when gate trigger is performed, to avoid a measurement error, a protection time Δ is provided at either side of the boundary so that the performance of the optical signal is measured from a time point at which t=kT+A to a time point at which t=(k+1)T−Δ. By configuring the optical recirculation loop as above and conducting the test, a long distance transmission is made possible with a small number of test samples.
However, the conventional unidirectional optical recirculation loop transmitting device described with reference to FIGS. 1 and 2 has a problem that an optical signal proceeding in the opposite direction cannot be generated. That is, the conventional transmitting device can be used for a unidirectional optical recirculation loop transmitting device, but it cannot be used for a bidirectional optical recirculation loop transmitting system.